Add multi-user auth, background generation, and chat UX improvements

Phase 5: Multi-user authentication with session cookies, bcrypt passwords,
first-user-is-admin pattern, per-user data isolation, backup/restore,
Docker Swarm production stack with secrets and network isolation.

Phase 5.1: Chat UX improvements:
- Background generation architecture (GenerationBuffer + asyncio task)
  with SSE fan-out, reconnect support, and periodic DB flushes
- LLM-generated conversation titles (first exchange + every 10th message)
- Stop generation button with cancel_event and partial content preservation
- Relative timestamps in sidebar (5m ago, 3h ago, then dates)
- Empty chat auto-cleanup on navigation away
- Save-as-note uses LLM for title generation, tags notes with "chat"
- Summarize-as-note also tags with "chat"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/stores/auth.ts

@@ -0,0 +1,63 @@
+import { ref, computed } from "vue";
+import { defineStore } from "pinia";
+import { apiGet, apiPost } from "@/api/client";
+import type { User, AuthStatus } from "@/types/auth";
+
+export const useAuthStore = defineStore("auth", () => {
+  const user = ref<User | null>(null);
+  const loading = ref(true);
+  const hasUsers = ref(true);
+
+  const isAuthenticated = computed(() => user.value !== null);
+  const isAdmin = computed(() => user.value?.role === "admin");
+
+  async function checkAuth() {
+    loading.value = true;
+    try {
+      user.value = await apiGet<User>("/api/auth/me");
+    } catch {
+      user.value = null;
+    } finally {
+      loading.value = false;
+    }
+  }
+
+  async function checkHasUsers() {
+    try {
+      const data = await apiGet<AuthStatus>("/api/auth/status");
+      hasUsers.value = data.has_users;
+    } catch {
+      hasUsers.value = true;
+    }
+  }
+
+  async function login(username: string, password: string) {
+    user.value = await apiPost<User>("/api/auth/login", { username, password });
+  }
+
+  async function register(username: string, password: string, email?: string) {
+    user.value = await apiPost<User>("/api/auth/register", {
+      username,
+      password,
+      email: email || undefined,
+    });
+  }
+
+  async function logout() {
+    await apiPost("/api/auth/logout", {});
+    user.value = null;
+  }
+
+  return {
+    user,
+    loading,
+    hasUsers,
+    isAuthenticated,
+    isAdmin,
+    checkAuth,
+    checkHasUsers,
+    login,
+    register,
+    logout,
+  };
+});